Many municipal sewage treatment plants in China are used as disposal sites of wastewater indirect emission sources, to receive various wastewaters, in particular industrial wastewaters with increasingly complicated components. The 2007 Environmental State Report of Zhejiang Province shows that only 58.8% of 50 municipal sewage treatment plants in normal operation in the province meet the discharge standards, and the main pollution factors beyond the standards are ammonia nitrogen and total phosphorus. The industrial wastewaters without pretreatment usually have a load of nitrogen and phosphorus exceeding the processing capacity of sewage plants, interfering the normal operation of sewage treatment devices, so that the sewage treatment plants become an important source of nitrogen and phosphorus in water bodies. Hence, wastewater from various sources usually should be pretreated to meet the requirements of sewage treatment plants, especially meet the influent requirements of sewage biotreatment plants.
At present, the frequently used wastewater pretreatment method is dilution method, in which when pollutant levels in wastewater exceed their tolerable limits for biotreatment, a simple dilution method can be adopted to decrease the pollutant levels below their limits so as to ensure the normal operation of biotreatment. The most simple and economic method is wastewater dilution method, i.e., mixing different wastewaters and/or sewages together to dilute each other so as to reduce pollutant levels below their tolerable limits. In some cases, the effluent water during the process or clear water can also be used for dilution. However, the pollutant levels of wastewater and/or sewage usually are very high, so that it is difficult to reduce the pollutant levels below their tolerable limits even by the mixing. In addition, the use of effluent water during the process or the clear water for dilution will cause a significant increase of treatment cost.
In addition, many sewage treatment plants usually meet in practical operation the following problems: (1) insufficient amount of sewage feed, mainly resulted from advanced design and failures in sanitary sewer system, which may influence the operation of sewage treatment apparatus; (2) unstable quality of sewage feed, mainly resulted from draining industrial wastewater into sanitary sewer system and living habit changes caused by holidays and seasonal variation, which may form shock load to influence sewage treatment effects; (3) carbon-source deficiency, as a common problem of sewage treatment plants, mainly attributed to modern living habits, which may result in unbalance of nutrients in organisms and influence the removal effects of nitrogen and phosphorus. Facing these challenges, conventional activated sludge methods usually have the following drawbacks: (1) low biomass concentration in aeration basin; (2) poor ability in resisting shock load of quality and amount of sewage feed; (3) easy sludge bulking; (4) high yield of sludge; (5) high cost in construction and operation, and large floor space, etc.
Hence, novel wastewater pretreatment methods to make the pretreated wastewater more suitable for biotreatment methods are still in need.